Field
Aspects of the present disclosure relate to air-to-ground communication systems, and more particularly to an air-to-ground communications system adapted for use with an airborne mobile platform that accomplishes soft hand offs between terrestrial base transceiver stations in a cellular network while the mobile platform is in flight.
Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. Typically, such networks are terrestrial-based networks, however, in recent years, publicly accessible networks are being made available for passengers on commercial air transportation, e.g., airplanes and other aircraft.
Such services are typically known as air-to-ground (ATG) communication services, and may provide such services as broadband data, voice communication, and entertainment such as streaming movies or music. Although ATG services and networks are similar to currently deployed terrestrial cellular and other wireless networks, there are aspects of ATG networks that differ from these networks.
Typically, as aircraft fly across a geographic region, each aircraft is serviced by a particular base transceiver station (BTS) until signal quality, signal strength, or available bandwidth from that BTS is insufficient, at which time service is transferred to another BTS. Such a transfer is typically called a “handoff,” similar to handoffs that occur in terrestrial cellular networks for cellular devices (handsets, PDAs, etc.) when such devices are mobile.
Aircraft typically use a single transceiver having an antenna mounted on the undercarriage of the aircraft to communicate with the BTS. However, BTS antenna patterns are usually designed to service terrestrial customers, and the beam patterns at a given BTS are usually not arranged to service ATG communications traffic.
Further, merely replicating the terrestrial cellular beam patterns around the aircraft in an omnidirectional pattern would provide insufficient signal strength and capacity to service the thousands of aircraft and potentially hundreds of thousands of users in such an ATG system.
Further, typical cellular and mobile devices use antenna patterns that transmit power in all directions, and if such antennas had high enough gain patterns to communicate directly with a terrestrial BTS, such transmissions would cause interference into all BTS sites within a line of sight of transmission of the cellular device. The line-of-sight transmission for a device increases when the transmitter/antenna pattern is at altitude, thus further complicating the interference problem. Increased interference also reduces data bandwidth, which creates lower data throughput in an ATG system that can use cellular telephones and other mobile devices directly.
As such, the antennas used in ATG systems are typically “directional” antennas, where the antenna on the aircraft directs the outgoing transmission in certain directions, and the BTS antenna also directs the transmission power in the direction of aircraft in the BTS service area. The aircraft antenna receives the omnidirectional transmissions from the cellular telephones on board, and the aircraft antenna directs these transmissions toward a specific BTS antenna, which reduces interference and increases data throughput.
Once the aircraft begins leaving a particular BTS service area, the aircraft service is handed off to another BTS in order to maintain communication with the devices on board that aircraft. Such a handoff should occur before communication is lost with the serving BTS to ensure continuous communication channels for the devices on board. Such communication channels are difficult to maintain without interaction between the aircraft antenna and the BTS antennas, because signal strengths and signal quality are typically not known.